1. Field of the Invention
The present invention relates generally to printing devices and, more particularly, is directed to a printing device for producing a hard copy of a picture displayed on the screen of a television receiver.
2. Description of the Prior Art
Various systems, such as the "TELETEXT" and "VIEWDATA" systems, are known in which news, weather and other information are transmitted, for example, during the vertical blanking period of a television broadcast or through a telephone circuit, to a television receiver where such information is displayed. With these systems, each number, letter or other symbol is converted to a code signal and then transmitted to the receiving end of the system where the code signal is decoded to the original number, letter or other symbol and then displayed on the screen of the television receiver. For example, for the letter "A", a corresponding code signal "41" may be transmitted to a character generator at the receiving end which, in response thereto, generates a luminance signal corresponding to the letter "A" and supplies the same to the television receiver. Accordingly, the letter "A" is displayed on the screen of the television receiver.
With the above system, a printer can be connected to the television receiver for printing a hard copy of the picture, that is, the combination of numbers, letters or other symbols, which are displayed on the screen of the television receiver. It should be appreciated, however, that a printer cannot operate at the speed at which the luminance signal is supplied to the television receiver. Since the picture displayed on the screen of the television receiver is a still picture, that is, it remains on the screen long enough for the viewer to read the information, the luminance signal is sampled with a suitable sampling frequency to enable the printer to follow the luminance signal as the corresponding numbers, letters or other symbols are being displayed on the screen of the television receiver so as to print the sampled output thereof and thereby produce the desired hard copy.
Generally, the numbers, letters or other symbols displayed on the screen of the television receiver are formed from a plurality of pictoral elements, which will hereinafter be referred to as full or normal dots. In order to more clearly display the numbers, letters or other symbols on the screen, a plurality of half dots, having half the width of a normal or full dot, are added to the pictoral representation to provide a more rounded look to the numbers, letters or other symbols, thereby making the latter easier to see. However, various problems have arisen with the different control circuits used to control the printer. It is to be first noted that the printer is only adapted to print a full pictoral element on the hard copy in response to the sampled signal supplied thereto. Thus, with one known printer circuit, if a signal corresponding to a half dot of information displayed on the screen of the television receiver is sampled, the printer will print a full pictoral element on the hard copy. Accordingly, the numbers, letters and other symbols which are printed on the hard copy may become distorted.
In another proposed printer circuit, two separate sampling circuits and a logic circuit are provided to eliminate the sampled luminance signal corresponding to each half dot of information. However, because this circuit requires two sampling circuits and a logic circuit, the circuit is relatively complicated. Further, with this circuit, the sampled luminance signal corresponding to successive half dots of displayed information is interpreted by the logic circuit as corresponding to a full dot of information, so that the printer incorrectly prints a full pictoral element.